Backplane buses provide connections between cards in a chassis. For example, a document entitled “PHASE—Flexibility and Modularity from STM-1 to STM-16” available over the Internet at http://home.wtal.de/jga/phase.pdf states that the type and features of PHASE network elements are determined by the type and number of the hardware and software modules used. The following basic types of network element are possible: Terminal Multiplexer (TM), Add/Drop Multiplexer (ADM), Local Cross Connect (LXC), Line Regenerator (LR). These basic types can be configured for the STM-1, -4 and -16 levels of the SDH hierarchy and for the VC-12 and VC-4 connectivity levels. This document further states that “Data traffic between the plug-in units is handled via bus systems in the backplane of the shelf to achieve the maximum possible flexibility in configuring the plug-in units in a network element.”